The Fundamental Difference Between Starch and Glycogen
To understand what stores starch in humans, it is essential to first clarify a common misconception. Starch is the primary energy storage polysaccharide in plants, whereas glycogen is the equivalent in animals and humans. When humans consume starchy foods like potatoes or grains, our digestive system breaks down the complex starch molecules into simpler glucose units. It is this glucose that the body then processes and stores, not the starch itself.
The storage form of glucose in the human body is glycogen, a highly branched polysaccharide. This branching structure allows for a very rapid breakdown of glycogen back into glucose when the body needs a quick source of energy. This is particularly important for high-intensity physical activity or during periods of fasting.
Where Glycogen is Stored in the Human Body
The body primarily stores glycogen in two key locations: the liver and the muscles. Although glycogen is also found in smaller amounts in other tissues like the kidneys and even certain cells in the brain, the liver and skeletal muscles are the main reservoirs.
Liver Glycogen: The Body's Central Glucose Reserve
Approximately 100-120 grams of glycogen are stored in the liver of an adult. The liver's role is to act as a central hub for glucose regulation, releasing stored glucose into the bloodstream to maintain stable blood sugar levels for the entire body, especially the brain. This process, called glycogenolysis, is crucial during periods of fasting or when blood glucose levels drop. The liver is able to convert glucose-6-phosphate back into free glucose because it possesses the necessary enzyme, glucose-6-phosphatase.
Muscle Glycogen: Fuel for Physical Activity
Skeletal muscles store the largest proportion of the body's glycogen, holding around 400 grams in an average adult. Unlike the liver, muscle cells primarily use their glycogen stores for their own energy needs, particularly during exercise. Muscle cells lack the enzyme glucose-6-phosphatase, which means they cannot release their glucose stores into the general bloodstream to support other organs. This functional distinction highlights the specialized roles of these two glycogen reservoirs.
How Excess Carbohydrates are Handled
When the body consumes more carbohydrates than it needs for immediate energy or for filling its glycogen stores, the excess glucose is converted into fat for long-term storage. This process, known as lipogenesis, primarily takes place in the liver and adipose tissue.
The Journey from Starch to Glycogen
The conversion process begins in the mouth, where salivary amylase starts to break down starch. The process continues in the small intestine, where pancreatic amylase completes the digestion, yielding glucose. The glucose is then absorbed into the bloodstream and travels to the liver. There, under the influence of the hormone insulin, glucose is converted into glycogen in a process called glycogenesis. When glucose is needed, the hormone glucagon signals the liver to break down glycogen back into glucose.
The Role of Key Hormones
The regulation of glycogen metabolism is primarily controlled by two pancreatic hormones: insulin and glucagon. Insulin is released when blood glucose levels are high, promoting the uptake of glucose by cells and its storage as glycogen. Conversely, glucagon is released when blood glucose levels are low, triggering the breakdown of glycogen to release glucose. This delicate balance ensures that the body's energy needs are met consistently.
A Comparison of Starch and Glycogen
| Feature | Starch (in Plants) | Glycogen (in Humans) |
|---|---|---|
| Function | Long-term energy storage | Short-term energy storage |
| Storage Location | Roots, seeds, leaves | Liver, muscles, kidneys |
| Structure | Less branched (amylose) or more branched (amylopectin) | Highly branched |
| Digestibility | Must be broken down by amylase | Can be broken down more rapidly |
| Reactivity with Iodine | Turns dark blue/black | Gives a reddish-brown color |
| Mobilization | Slower mobilization | Faster mobilization for quick energy |
Conclusion
In summary, the human body does not store starch. Instead, it digests dietary starch into glucose, which is then stored in the form of glycogen. The liver and muscles are the primary storage sites for this glycogen, serving as the body's short-term energy reserve. Liver glycogen is used to regulate blood sugar for the entire body, while muscle glycogen provides fuel specifically for muscle contraction during physical activity. This intricate system of digestion, conversion, and storage, managed by hormones like insulin and glucagon, is fundamental to human energy metabolism and overall health. For further details on how the body uses energy, refer to the National Institutes of Health.
The Process of Glycogen Synthesis
- Glucose Uptake: After a meal, blood glucose levels rise, and glucose enters liver and muscle cells.
- Phosphorylation: The enzyme hexokinase (in muscles) or glucokinase (in the liver) adds a phosphate group to glucose, trapping it inside the cell as glucose-6-phosphate.
- Isomerization: Glucose-6-phosphate is converted to glucose-1-phosphate by the enzyme phosphoglucomutase.
- Activation: An enzyme then activates glucose-1-phosphate to form UDP-glucose.
- Elongation and Branching: Glycogen synthase adds UDP-glucose to a growing glycogen chain, while a branching enzyme creates branches to increase storage capacity.
The Breakdown of Glycogen
- Hormonal Signal: When blood glucose levels drop, the pancreas releases glucagon.
- Glycogen Phosphorylase Activation: Glucagon stimulates the enzyme glycogen phosphorylase, which begins to cleave glucose units from the glycogen chains.
- Debranching: A debranching enzyme is required to deal with the α-1,6 linkages at the branch points, transferring glucose units to the main chain.
- Release of Glucose: In the liver, the enzyme glucose-6-phosphatase removes the phosphate group from glucose-6-phosphate, allowing free glucose to enter the bloodstream. Muscle cells, lacking this enzyme, use the glucose-6-phosphate locally for energy.
